In the process of thus depositing the cathode metal upon the substrate, a trough is eroded in the cathode assembly and which trough corresponds to the enclosed or annular shape of the magnetic field generated by the Planar Magnetron mechanism. Once the depth of the trough extends through the original thickness of the metal cathode, it is necessary to replace the cathode assembly even though only a portion of the cathode volume has been consumed. Heretofore, such cathode assemblies must be replaced when only between 20% and 30% of the cathode metal has been consumed.
By way of example, the cost of a typical rectangular cathode assembly made of pure chromium and being 12" wide .times. 29" long and having a thickness of 1" is in the range of $6,500. As noted, in the past such assembly had to be replaced even though 70% to 80% of the cathode volume is unconsumed. From a user's viewpoint, such low volumetric yield is undesirable and progressively more so as more costly cathode materials, such as gold, platinum, silver or palladium, are used in the sputtering or vapor deposition process.
It is the purpose of the present invention to greatly increase the useful life of a consumable cathode assembly used in the sputtering or vapor deposition process by reorienting the unconsumed metal volume of the assembly so as to be available for deposition without replacement of the original cathode assembly components. More specifically, the present invention relates to a consumable cathode assembly and a method for making such which, in general, doubles the volume of metal cathode material available for deposition and, therefore, reduces the consumable metal cost generally by one-half.
As noted, one type of deposition mechanism in which the consumable metal cathode assembly of the present invention is utilized is referred to as a Planar Magnetron. The mechanism includes a consumable metal cathode disposed in a vacuum chamber into which a non-oxidizing gas, such as argon, is introduced and within which chamber a substrate material is located for the deposition thereon of a thin film of the cathode metal. An annular magnetic coil is positioned immediately behind the cathode assembly. The magnetic coil is energized to generate a closed loop magnetic field across the cathode material. The magnetic field ionizes the non-oxidizing gas and accelerates the charged atoms thereof within the magnetic field, causing such atoms to collide with and thereby vaporize the surface of the cathode metal within the vacuum chamber. The vaporized cathode metal is propelled linearly away from the cathode assembly and condenses as an evenly distributed film on the proximately disposed substrate material.
The area of erosion of the cathode assembly both in width and length corresponds generally to the annular magnetic field with the width being defined by the magnetic field lines across the cathode assembly generally parallel to the surface thereof. The depth of the erosion into the cathode is determined by the current density across the cathode and over a period of time; this creates an eroded trough having a curved V-shape or double-convex cross section. At such time as the bottom of the eroded trough approaches the original thickness of the cathode, it is necessary to replace the cathode assembly even though only 20% to 30% of the cathode volume has been consumed. In other words, less than 1/3 of the original cathode volume is consumed at the time replacement has heretofore been necessary.
With the present invention, the cathode assembly is formed of a plurality of individual target tiles arranged in end-to-end abutting relationship so as to form a continuous and enclosed length of consumable metal. More specifically, the individual target tiles include two pairs of end targets transversely connecting two longitudinal rows of abutting targets altogether forming a closed and continuous loop of consumable metal overlaying the generated magnetic field. In use, the individual targets are eroded so as to define a continuous annular track or trough having a curved V-shape or double-convex cross section with the deepest part of the trough being generally intermediate the width of each target tile. The continuous and annular eroded trough includes linear or straight sections formed through the two longitudinal tile rows and curved or arcuate sections formed through the end tiles.
At such time as the deepest part of the trough approaches the original thickness of the target tiles, the cathode assembly must be replaced. In accordance with the invention, each target tile of the longitudinal rows is severed along the deepest part of the trough and reassembled with the thickest or least eroded parts of the severed target segments in transversely abutting relationship. At the same time, each of the four end tiles is rotated through 180.degree. upon its base to position the non-eroded portion thereof in abutting relationship with the endmost severed and reassembled targets. The severed, reassembled and rotated tiles thus form a new cathode assembly wherein the thickest or least eroded parts of each target tile are reoriented, in effect, to fill in the volume of the eroded trough to provide a new and continuous closed length of non-eroded cathode metal. In this manner the useful life of the consumable cathode assembly is doubled, reducing generally by one-half the replacement cost of such assembly.
The details of the invention are set forth in the drawings and following description.